176 



THE SKELETON. 



are very variable. It is not always present ; sometimes it is situated in the 

 occipital bone or in the suture between the temporal and the occipital. The 

 mastoid portion is continued below into a conical projection, the mastoid process, 

 the size and form of which vary somewhat. This process serves for the attachment 

 of the Sterno-mastoid, Splenius capitis, and Trachelo-mastoid muscles. On the 

 inner side of the mastoid process is a deep groove, the digastric fossa, for the 

 attachment of the Digastric muscle ; and, running parallel with it, but more in- 

 ternal, the occipital groove, which lodges the occipital artery. The internal surface 

 of the mastoid portion presents a deep, curved groove, the fossa sigmoidea, which 

 lodges part of the lateral sinus ; and into it may be seen opening the mastoid fora- 

 men. A section of the mastoid process shows it to be hollowed out into a number 

 of cellular spaces, communicating with each other, called the mastoid cells ; they 

 open by a single or double orifice into the back of the tympanum, are lined by a 

 prolongation of its lining membrane, and probably form some secondary part of 

 the organ of hearing. The spaces at the upper and front part of the bone near 

 the opening into the tympanum are large and irregular, and contain air. They 

 diminish in size toward the lower part of the bone, those situated at the apex of 

 the mastoid process being quite small and usually containing marrow (Fig. 140). 



TENSOR TYMPANI. 



'roc. cochleariformis. 

 'Eustachian tube. 



Carotid canal. 



Carotid canal. 

 Fenestra rotunda. 



Pyramid! 



Styloid process. 

 Aquxductus Fallopii. 



'Harrow cells. 



FIG. 140. Section through the petrous and mastoid portions of the temporal bone, showing the communi- 

 cation of the cavity of the tympanum with the mastoid cells. 



The mastoid cells, like the other sinuses of the cranium, are not developed 

 until after puberty ; hence the prominence of this process in the adult. 



In consequence of the communication which exists between the tympanum and mastoid 

 cells, inflammation of the lining membrane of the former cavity may easily travel backward to 

 that of the mastoid cells, leading to caries and necrosis of their walls and the risk of transfer- 

 ence of the inflammation to the lateral sinus or encephalon. 



Borders. The superior border of the mastoid portion is broad and rough, its 

 serrated edge sloping outward, for articulation with the posterior inferior angle of 

 the parietal bone. The posterior border, also uneven and serrated, articulates 

 with the inferior border of the occipital bone between its lateral angle and jugular 

 process. 



The Petrous Portion (TTS^OC, a stone), so named from its extreme density and 

 hardness, is a pyramidal process of bone wedged in at the base of the skull 

 between the sphenoid and occipital bones. Its direction from without is inward, 



